Stage 4: lobby & social (matchmaking, friends, blocks, chat+nudge, invitations, profile, email, multi-player drop-out)

Engine: multi-player drop-out-and-continue with a per-game tile disposition (remove default / return), resigned seats skipped and excluded from the win, leaver rack never revealed; 2-player behaviour unchanged.

New domains (service/store, no HTTP yet): internal/social (friend request/accept graph, per-user blocks, per-game chat with nudge as a message kind, content filter via mvdan.cc/xurls/v2 + leet/separator normaliser + phone heuristic) and internal/lobby (in-memory variant-keyed matchmaking pool, friend-game invitations invite->accept with lazy 7-day expiry). account gains profile editing and the email confirm-code flow (Mailer seam: SMTP or log mailer).

Migration 00003_social.sql + regenerated jet. main wires the new services into the server (accessors for the Stage 6 handlers); robot substitution stays in Stage 5, REST/stream/push in Stage 6/8. Docs (PLAN, ARCHITECTURE, FUNCTIONAL+ru, TESTING, README) updated.

backend/cmd/backend/main.go

@@ -1,10 +1,10 @@
 // Command backend is the Scrabble platform's internal domain service. It boots
 // the OpenTelemetry runtime, opens the Postgres pool and applies migrations,
 // loads the dictionaries into the engine registry, warms the session cache,
-// constructs the game domain and starts its turn-timeout sweeper, then serves the
-// HTTP listener with the infrastructure probes and the /api/v1 route-group
-// skeleton. Domain HTTP endpoints are added with the gateway in a later stage
-// described in PLAN.md.
+// constructs the game domain and starts its turn-timeout sweeper, constructs the
+// lobby and social domains, then serves the HTTP listener with the infrastructure
+// probes and the /api/v1 route-group skeleton. Domain HTTP endpoints are added
+// with the gateway in a later stage described in PLAN.md.
 package main
 
 import (
@@ -21,9 +21,11 @@ import (
 	"scrabble/backend/internal/config"
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/game"
+	"scrabble/backend/internal/lobby"
 	"scrabble/backend/internal/postgres"
 	"scrabble/backend/internal/server"
 	"scrabble/backend/internal/session"
+	"scrabble/backend/internal/social"
 	"scrabble/backend/internal/telemetry"
 )
 
@@ -95,20 +97,46 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	}
 	logger.Info("session cache warmed")
 
-	games := game.NewService(game.NewStore(db), account.NewStore(db), registry, cfg.Game, logger)
+	accounts := account.NewStore(db)
+	games := game.NewService(game.NewStore(db), accounts, registry, cfg.Game, logger)
 	go games.RunSweeper(ctx, cfg.Game.TimeoutSweepInterval)
 	logger.Info("game turn-timeout sweeper started",
 		zap.Duration("interval", cfg.Game.TimeoutSweepInterval))
 
+	// Stage 4 lobby & social domains. They have no active driver yet — their REST
+	// and stream surface is added with the gateway in Stage 6 — so they are handed
+	// to the server (like the route groups) for the handlers to come.
+	mailer := newMailer(cfg.SMTP, logger)
+	emails := account.NewEmailService(accounts, mailer)
+	socialSvc := social.NewService(social.NewStore(db), accounts, games)
+	matchmaker := lobby.NewMatchmaker(games)
+	invitations := lobby.NewInvitationService(lobby.NewStore(db), games, accounts, socialSvc)
+	logger.Info("lobby and social domains ready")
+
 	srv := server.New(cfg.HTTPAddr, server.Deps{
 		Logger:        logger,
 		DB:            db,
 		PingTimeout:   cfg.Postgres.OperationTimeout,
 		SessionsReady: sessions.Ready,
+		Social:        socialSvc,
+		Matchmaker:    matchmaker,
+		Invitations:   invitations,
+		Emails:        emails,
 	})
 	return srv.Run(ctx)
 }
 
+// newMailer builds the confirm-code mailer: an SMTP relay when a host is
+// configured, otherwise the development log mailer (the code is logged, not sent).
+func newMailer(cfg account.SMTPConfig, logger *zap.Logger) account.Mailer {
+	if cfg.Host == "" {
+		logger.Info("email: using log mailer (BACKEND_SMTP_HOST unset)")
+		return account.NewLogMailer(logger)
+	}
+	logger.Info("email: using SMTP relay", zap.String("host", cfg.Host))
+	return account.NewSMTPMailer(cfg)
+}
+
 // newLogger builds a production JSON logger at the given level.
 func newLogger(level string) (*zap.Logger, error) {
 	var lvl zap.AtomicLevel